Recent developments in fracturing technology have led to significant increases in the recovery of oil and gas deposits, such as natural gas deposits bound in tight shale formations. Hydraulic fracturing technologies are the primary means for well stimulation to increase the permeability of the reservoir and enhance recovery of oil and gas. During hydraulic fracturing, a fluid is pumped down into the wellbore at a pressure sufficient to cause fractures in the reservoir, and proppants suspended in the fracturing fluid are used to keep the fractures open and enable gas to subsequently flow into the wellbore. The fracturing fluid will often contain viscosifying or gelling agents to increase the viscosity so that proppants can be suspended in the fracturing fluid during the process.
Although in-place, unconventional resources are substantial, recovery efficiencies are commonly low in these reservoirs. Although improved technologies have increased recovery percentage, significant performance gains are still to be made. Additionally, the widespread proliferation of new gas wells and the use of modern drilling and extraction methods have been identified as a global conservation issue. The alternative to hydraulic fracturing is to drill more wells in an area, a solution that is often economically or geographically prohibitive. Several hurdles to significant technology advancements still remain.
In aqueous fracturing fluids, the proppants must be suspended using a combination of additives and turbulent flow. As flow naturally slows, the proppants have a tendency to settle, leading to reduced permeability and potential damage to surface equipment or long horizontal laterals. Conventional fracturing fluids are also not suitable for unconsolidated reservoirs because the formation material can re-consolidate around the proppants or the proppants can get embedded into the formation, leading to reduced permeability.
Long-term production also can suffer with known fracture methods. For example, as the well is produced, changes in pressure and fluids cause proppant shift, resulting in premature closure and rapidly declining production over the life of the well. Also, proppant debris generated during the fracturing process is mobilized over time, obstructing the pathway and resulting in loss of flow capacity for hydrocarbon. Relatedly, the gelling agents used to suspend the proppant ultimately block the pores and reduce the permeability of the reservoir.
Water use in known fracture methods also is a concern. A significant volume of water is required for fracture operations, and the balance is either lost to the formation as leak-off or returned as flowback water that must be treated. A growing number of challenges and limited options are associated with properly treating and disposing of hydrofracturing wastewater in light of strict regulations and high treatment costs
To address fracture efficiency, a host of fracturing fluid additives and proppants are under development and in practice. For slickwater fluids, known to be inherently poor proppant carriers, viscoelastic surfactant gel fluids are one of many gelling additives that have been considered. Principally, these fluids use surfactants in combination with inorganic salts to create ordered structures, which result in increased viscosity and elasticity. Engineered proppants are also increasing in utility, providing uniform, spherical packing and reducing the production of fines typically produced at high pressure and impact. Thus far, engineered proppants have been limited in utility because of their high cost relative to sand.
Non-aqueous fracturing fluids are gaining attraction for several reasons, including environmental and economic concerns surrounding flowback and produced water and potential for more efficient production through elimination of both aqueous phase trapping from capillary retention of the water, and residual gels and surfactant. Non-aqueous fluids include polymer/methanol-based, oil-based, and gas-based systems; each of these approaches has limitations, from the perspectives of cost, safety, and/or reliability. Oil- and alcohol-based fracturing fluids suffer from cost pressures and health and environmental safety concerns due to flammability and environmental impact. Pure and binary gas mixtures of CO2 and N2 have been employed to a limited extent to date. The main disadvantage of these fluids is safety (i.e., pumping a gas at high pressure), and proppant dispersion has been a challenge.
Improvements in additive and proppant technologies are also being employed to increase long-term production. Much emphasis has been placed on reducing polymer concentration and/or adding breakers to cleave the polymer chains after placement. Polymer residue tends to block the pore spaces between proppant particles and, due to fluid loss to the formation, polymer additives are left at higher concentrations, leaving a gelled mass. Encapsulated breakers and crosslinked fluids are being employed to improve performance without increasing concentration. Reducing proppant flowback after treatment is also under investigation. As greater fracture widths and higher proppant concentrations are becoming the norm, up to 20% or even 50% of proppant can be produced back resulting in premature closure or termination of production. Resin-coated proppant that can be cured after placement for flowback control has gained in popularity. Although less expensive than engineered ceramic proppants, the utility of resin-coated proppant has been limited because of concerns around hydrocarbon permeability through the resin and the higher cost as compared with uncoated sand. These limitations have led to the use of resin-coated proppants at just the end of each fracture stage. Fiber-laden fracturing slurries also have been used to improve fracture geometries and enhance production. Fibers and fiber bundles aid in proppant suspension and can be made degradable to reduce concerns over residue blocking pores.
A wide variety of processes, technologies, and management strategies are under development and in use to address the foregoing problems. One proposal for stimulating subterranean formations has been through use of a fracturing fluid comprising a permeable cement composition. See, for example, U.S. Pat. No. 7,044,224, which proposes the use of cements formulated with degradable polymer additives or dehydrated salts that dissolve over time as the salts hydrate. Removal of such additives is intended to form natural voids in the cement matrix; however, unless the proposed voids are interconnected, they do not contribute to enhanced production. Additionally, increases in void volume must be carefully balanced with mechanical strength, as too much porosity can cause collapse of the structural unit. Accordingly, there remains a need for further materials useful in fracture fluids.